White light emitting diodes (LEDs) are commonly produced by converting a portion of the output of a blue LED to longer wavelengths, and combining the remaining blue light with the down-converted light to make white light. The down-conversion is usually achieved using a phosphor that coats the emitting surface of the blue LED die. This configuration of down-converting phosphor and LED die is referred to as a direct phosphor configuration.
Direct phosphor configurations can have several drawbacks. The configuration increases both the LED and the phosphor temperature, which can reduce both the efficiency and the lifetime of each of the components. Further, about half of the light emitted by the phosphor can re-enter the LED die, and due to the relatively low reflectivity and absorption in the die, the system efficiency can be further reduced.
A promising alternative to the direct phosphor configuration is the remote phosphor configuration. Remote phosphors fall into two broad categories: reflective and transmissive. Transmissive remote phosphor LEDs have one or more LED die that illuminates a phosphor containing layer, where the phosphor layer is positioned at distance from the LED die. Normally, the LED die are spaced from each other and are bonded to a highly reflective substrate. Reflective remote phosphor configurations have one or more die illuminating a phosphor coated reflective surface, where the combination of light reflecting from the phosphor layer along with the converted light form white light.
Both the reflective and transmissive remote phosphor configurations substantially increase the emitting area of the LED, increase the amount of phosphor, and thereby increase etendue, which is undesirable in applications where the light will be directed with lenses and mirrors.